Assessment of spatial and temporal soil loss in and out of Lesotho using Rusle Model and GIS
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Lesotho is one of the major sources to rivers that drain the southern part of southern Africa through Senqu and Sabi-Sand River. However severe soil erosion is the major environmental problem that has left the landscape dissected with deep gullies and dongas. Fertile soil, that should be supporting food production is washed in billions of tons as sediments in lakes, reservoirs and rivers reducing the aesthetic appearance of water bodies with high sediment loads leading to low agricultural productivity and high costs of water treatment for water supply. There is inadequate current information as to how much soil has been lost or is being lost in and out of Lesotho. Studies of the last four decades are still cited in policy formulations while the earth and its constituents are dynamic and ever changing. The objective of this study was to assess the spatial and temporal (1986, 1997 and 2009) soil loss in and out of Lesotho using the RUSLE model and GIS techniques. The annual soil loss computed from RUSLE model was compared with the observed sediment yield of gauged rivers that are leaving Lesotho for the duration of the study (1980 – 2012). Data used consisted of DEM of 90m x 90m resolution to calculate the slope length-steepness factor (LS) and the soil map was used to calculate the soil erodibility factor (K). The temporal factor, used to compute rainfall erosivity (R) for 1986, 1997 and 2009. Then Landsat images were used to come up with the land cover management factor and conservation practices for 1986, 1997 and 2009. Flow and sediment data from 1980 to 2012 were used to determine relationships, trends and total amounts of soil that left Lesotho for the three decades. To identify critical areas/ hotspots for future management from the results predicted by RUSLE model and observed sediment yields. The results of land classification for 1986, 1997 and 2009 indicate a steady increase of area under rock outcrop respectively from 24.4% to 38.4% and 42.42%, however a fluctuating decrease of percentage area under Alpine grassland and cropland. The result from the RUSLE model indicates that average annual loss of 9,079,499.29 ton ha-1 year-1, 7,785,643.78 ton ha-1 year-1 and 26,711,616.90 ton ha-1 year-1 for 1986, 1997 and 2009 respectively. The observed sediment loads result indicates that 12,977,905.35 ton year-1 was lost 1986, 10,148,969.26 ton year -1 in 1997 and 21,151,804.37 ton year-1 in 2009..The north eastern highland and southern lowland districts are identified critical areas/ hotspots for future management. There is a strong relationship between the observed and predicted soil loss for the RUSLE model. The study discovered that there is a positive the correlation between the RUSLE model results and the observed sediments loads. In conclusion there is a tremendous soil loss in in Lesotho both spatially and temporally.